The present invention relates to random propylene copolymers containing copolymerized C.sub.2 -C.sub.10 -alk-1-enes obtainable by polymerization of propylene and C.sub.2 -C.sub.10 -alk-1-enes in the presence of a Ziegler-Natta catalyst system comprising a titanium-containing solid component a) comprising a compound of magnesium, a halogen, silica gel as support and a carboxylic ester as electron donor compound, and also, as cocatalysts, an aluminum compound b) and a further electron donor compound c), where propylene and the C.sub.2 -C.sub.10 -alk-1-enes are polymerized with one another at from 50 to 100.degree. C. at pressures in the range from 15 to 40 bar and mean residence times of from 0.5 to 5 hours and the support used in the titanium-containing solid component is a silica gel which has a mean particle diameter of from 5 to 200 .mu.m, a mean particle diameter of the primary particles of from 1 to 10 .mu.m and voids or channels having a mean diameter of from 1 to 10 .mu.m, which voids or channels have a macroscopic proportion by volume based on the total particle in the range from 5 to 20%.
In addition, the present invention relates to a process for preparing such random propylene copolymers and their use as films, fibers and moldings.
Catalyst systems of the Ziegler-Natta type are known, for example, from EP-B 014523, EP-A 023425, EP-A 045975 and EP-A 195497. These systems are used, in particular, for the polymerization of C.sub.2 -C.sub.10 -alk-1-enes and contain, inter alia, compounds of polyvalent titanium, aluminum halides and/or aluminum alkyls, and also electron donor compounds, in particular silicon compounds, ethers, carboxylic esters, ketones and lactones which are used both in connection with the titanium component and also as cocatalyst.
The Ziegler-Natta catalysts are customarily prepared in two steps. The titanium-containing solid component is prepared first and subsequently reacted with the cocatalyst. The polymerization is then carried out by means of the catalysts thus obtained.
Furthermore, U.S. Pat. No. 4,857,613 and U.S. Pat. No. 5,288,824 describe catalyst systems of the Ziegler-Natta type comprising not only a titanium-containing solid component and an aluminum compound but also organic silane compounds as external electron donor compounds. The catalyst systems thus obtained have a good productivity and give polymers of propylene having a high stereospecificity, ie. a high isotacticity, a low chlorine content and a good morphology, ie. a low fines content.
For some applications of propylene polymers, it is necessary for these to have a high stiffness and only a small proportion of xylene-soluble polymer particles. This is the case, for example, for food films produced from such propylene polymers.
The propylene polymers known from U.S. Pat. No. 4,857,613 and U.S. Pat. No. 5,288,824 do not meet these requirements to a sufficient extent.